The field of the invention is that of digital cellular mobile radio systems.
To be more precise, the invention concerns a method of transmitting a control signal by a base station of a digital cellular mobile radio system and a corresponding base station.
The invention applies in particular, but not exclusively, to transmitting BCCH signals in mobile radio systems of the following types: GSM 900 (Global System for Mobile communications operating in the 900 MHz band), DCS 1800 (Digital Cellular System operating at 1800 MHz) or PCS 1900 (Personal Communication System).
A digital cellular mobile radio system is usually implemented within a network of geographical cells through which mobile stations move. A base station is associated with each cell and a mobile station communicates via the base station associated with the cell in which it is located.
Each base station uses a number of signals (also called carriers) at predetermined frequencies on the downlink (from the base station to the mobile stations). One of these signals, called the BCCH (Broadcast Control Channel) signal, is a control signal carrying in particular (generally in the first time slot of each frame) a broadcast control channel BCCH specific to the cell and supplying all mobile stations with general information about the network and information about the broadcasting cell and adjacent cells.
The problem addressed here is transmission of the BCCH signal or more generally of any control signal by the base station. Because all of the cell must be covered, transmission of the signal must comply with a number of constraints. First of all, it must be isotropic (or quasi-isotropic). Also, it must be effected at a sufficiently high power, continuously and at a fixed frequency.
For the above constraints to be complied with, the BCCH signal is conventionally transmitted by an omnidirectional antenna associated with a power amplifier.
However, proposals have been made to replace omnidirectional antennas with antenna arrays in order to increase the size of cells and/or reduce base station transmit power levels. Antenna arrays of this kind form beams dynamically and therefore optimize use of resources.
Initially, only traffic signals have benefited from being transmitted on beams. In other words, each base station has had one or more antenna arrays for transmitting traffic signals and has retained an omnidirectional antenna for transmitting the BCCH signal. The constraints that transmission of the BCCH signal has to comply with are such that for a long time it has been thought that only an omnidirectional antenna could be used.
However, more recently, proposals have been made to transmit the BCCH signal using one or more antenna arrays. The number of antenna arrays is generally directly related to the number of sectors that the cells comprises, one antenna array covering each sector.
Patent document WO/9617486 proposes a base station comprising means for forming a plurality of fixed beams, each covering a part of the cell, and means for assigning the BCCH signal to each of the fixed beams in succession. In this way, transmission of the BCCH signal corresponds to coverage of the cell by cyclic scanning. The above document recommends that the BCCH signal is transmitted in each beam for the duration of one time slot of the frame structure. In this way each beam receives the BCCH signal every N time slots where N is the total number of beams.
The solution proposed in the above patent document is not satisfactory.
First of all, although the BCCH signal is transmitted isotropically, this cannot be considered to be true broadcasting because of the long interval (N time slots) between two transmissions on the same beam.
What is more, because of this long interval, it is difficult for a mobile station to maintain accurate knowledge of the better adjoining base stations. Also, the mobile station may have moved between two successive pointings in its direction, which increases the probability of loss of the BCCH signal and the complexity of handover between sectors and/or between cells.